Chapter 5 Larval transport in three different marine ...shodhganga.inflibnet.ac.in/bitstream/10603/12641/10... · Larval transport in three different marine ecosystems 5.1 Introduction

Chapter 5

Larval transport in three different marine ecosystems

5.1 Introduction

For the domains discussed in chapter 4, fish and shell fish larval transport has been stud-

ied using hydrodynamic and particle tracking models. Management of fisheries along the

coastal waters of India are carried out on the archetype that local fisheries is well mixed

with the open waters, and closed areas are enforced as a protective measure for prospec-

tive nursery areas of fish [Singh, 2003]. But, there is always the possibility of a self

recruitment of fish taking place in these waters which was not established in any of the

earlier field studies. The irregular coastline of WCI along with its shoals and reefs may

trap water and inert particles. A sufficient retention time in a basin could retain fish lar-

vae [Lobel and Robinson, 1986], zooplankton [Boicourt, 1982; Sammarco and Andrews,

1988; Murdoch, 1989; Thiebaut et al., 1994], phytoplankton [Roff et al., 1979] and other

neutrally buoyant material [Wolanski and Hamner, 1988; Black et al., 1990] in this area.

The pelagic larval phase of fishes/ shell-fishes are responsible for their dispersion or reten-

tion [Cowen and Sponaugle, 2009], and during this phase larvae are considered as "poor

swimmers" [Leis et al., 2006a] when the hydrodynamic forcing on the larvae exceeds its

swimming ability, but there are proven cases where larval behaviour has influenced dis-

persal trajectories [Chia et al., 1984; James et al., 2002; Cowen et al., 2006; Aiken et al.,

2007].

The scale and predictability of measured fish larval dispersion or retention remain un-

known largely due to the difficulty in measuring dispersion in open marine environments.

Utilization of high-resolution biophysical models in estimating dispersal distances or re-

tention time is advantageous as the models allow multiple releases of virtual eggs/ spawn,

thus making each individual simulation equivalent to numerous observations of dispersal

event. These virtual observations provide information about expected variability in hydro-

dynamics and allow construction of a connectivity matrix [Cowen and Sponaugle, 2009].

Larval transport in three different marine ecosystems 82

A common strategy employed in this kind of model is to predict the maximum likelihood

of retention of investigated species based on habitat attributes [Guisan and Zimmermann,

2000; Moisen et al., 2006; Elith and Graham, 2009]. Numerical modelling of fish eggs

dispersion at the Patos Lagoon estuary in Brazil was carried out by Martins et al. [2007]

using similar methodology.

No work has been carried out so far in the coastal waters of India to determine the influ-

ence of physical forcing on fish/shell-fish larvae under which they are widely dispersed

or locally retained. One of the objectives of this study is to find out whether the abundant

fish population in the GoK and Mangalore as well as the barnacle population in Mandovi-

Zuari estuary is the manifestation of self recruitment by the adult population trapped due

to hydrodynamic or geographical barriers. In this study, eggs/ spawn are released as inert

conservative particles from their representative spawning sites under a range of hydro-

dynamic and associated dispersion processes unique to the study areas to simulate the

spreading of eggs/ spawn and transport of larvae. The percentage likelihood of retention/

dispersal of larvae from spawning sites have been quantified.

5.2 Fish larval transport in a semi-enclosed basin: the

Gulf of Kachchh

5.2.1 Background

Mangrove and coral reef ecosystems are the spawning and nursery grounds for a majority

of fishes in the tropical coastal waters [Chittaro et al., 2004]. The last two decades wit-

nessed rampant destruction of coral reefs and mangrove ecosystem due to anthropogenic

pressures and climate change [Chittaro et al., 2004; Mumby et al., 2004]. Degradation of

these ecosystems resulted in reduced recruitment of fish worldwide [Rogers and Beets,

2001] and it is not very different in the GoK. GoK is famous for its fisheries potential

[Vijayalakshmi et al., 1993]. Establishment of industries close to the coast resulted in

destruction of flora and fauna, which are closely associated with the spawning and larval

emc cot IrE LONGITUDE

UWND[D=uwndseig995.2002] . VIYND[D=vwnd.sig995.2002] 10.0

Larval transport in three different matt, ecosystems DUE 20-NOV-2002 00 :00

(a)

GrE Trr Ten EWE LONGITUDE

OWND[D ■ uwnd.sig995.2002] . VWNIAD=vvrnd.sig995.2002] 10.0

(b)

83

Figure 5.1: Wind pattern in the GoK region: (a) during NE monsoon and (b) during pre monsoon season

rearing cycle of fishes [Vijayalakshmi, 2002]. Larvae are treated as passive particles, and

are released from the spawning sites which are decided based on a field survey in the

GoK.

5.2.2 Spawning behaviour of fish in GoK

The spawning sites surveyed for fish egg abundance (Table 5.1) were along the coasts,

and away from the influence of strong currents viz., mid-channel and eddies of the Gulf.

Particles released from sites A, B, E and F (Figure 5.3) experienced wider dispersal pattern

than at sites C and D. Hence, the sites A, B, E and F have been selected as spawning sites.

During NE monsoon, eggs are retained in the southern GoK due to predominant winds

from NNW (340°) (Figure 5.1) ruling out the importance of spawning sites at E and F.

Our field surveys also indicated higher egg abundance at sites B and F during NE and pre

monsoon seasons, and these sites are showing wider dispersal patterns (Table 5.1). Based

on these considerations, the spawning sites at B and F have been chosen for egg release.

Site A was selected for all the three seasons as particle trajectory revealed dispersal in all

the three seasons when particles were released and tracked from this site. Thus, particles

were released from two sites (F and A during pre monsoon and SW monsoon and B and

A during NE monsoon). Since egg abundance data were not available for SW monsoon


season, spawning sites were assumed to be the same as pre monsoon period. Primary

nursery areas are those areas in the estuarine system where initial post-larval development

takes place. These areas are usually located in the uppermost sections of a system, where

populations are uniformly very early juveniles. In this study, nursery areas are defined as

those areas where <30% likelihood of retention of larvae is simulated (Figure 5.4). The

numerical experiments show that these nursery areas are distributed in the ecologically

sensitive regions (Figure 1.3) along the GoK, except for the northwestern part of the Gulf.

5.2.3 Fish larval dispersion/retention

(i) Pre-monsoon: There was uniform dispersal of particles along the northern and

southern boundaries of GoK in the pre monsoon season (April), and less than 20%

of particles exited along the northern boundary to the open waters. The number of

exited particles is negligible in comparison to the spread along the boundaries of

GoK (Figure 5.2). The particles dispersed along the southern boundaries remained

within the domain. Thus, it can be inferred from the numerical experiment that the

fish eggs released along the southern boundaries of the gulf tend to remain in the

region, whereas the eggs from spawning sites in the northern gulf does not remain

within the gulf. The northwestern boundary of the gulf near spawning site F is a

crucial nursery ground during pre monsoon and SW monsoon seasons. But, this

area is not regulated for fishing. Fish egg abundance survey also corroborates the

numerical simulation.

(ii) SW monsoon: Particle trajectories were simulated for this season to observe the

influence of SW monsoon (June) on fish larval transport. The particles were dis-

persed from the spawning sites predominantly south of GoK. Those released in the

north being retained with larger larval retention on the reefs (Figure 5.2) in contrast

to a wider dispersal as seen during pre monsoon season. Less than 10% of larvae

exited the domain during the SW monsoon season. The westward movement of lar-

vae is restricted by the predominant SW winds, and hence only a small percentage

of larvae exited the domain. Similar to pre monsoon season, the particles dispersed


along the southern boundaries remained within the domain.

(iii) NE monsoon: The particle dispersal during the NE monsoon (November) followed

the pattern of previous seasons for the station close to Okha (site A). But, at station

close to B, the particle release showed retention in and around the release site with-

out much dispersal (Figure 5.2). Less than 10% of the particles exited the domain

as in the case of SW monsoon season. Retention of larvae is higher at the spawning

site B beacause B is surrounded by shoals, where currents are relatively weaker— a

favourable condition for accumulation.

0

50 100 150 (kilometer)

11/21/02 20:40:00

0 50 100

0 50 100

150

(kilometer)

(kilometer)

FINAL DAY 04/21/02 20:40:00

07/02/02 22:00:00

Concentratton ikgbral

Above 2s00:

Ei'I7.70oss 1.4e 005 1.0.40!

1.2.005 , IA.

1,4105 1.2e401

ME Re-o6- le00!

MI G1.000 - RoiJOE

440G- 6.41

1.11 2A001. 4e$231

IM O. 2o nol ____ mow

DAY 10


Figure 5.2: Simulation of larval dispersal on day 1, 5, 10 and 16 in the GoK: (a) Pre-monsoon, (b) SW monsoon and (c) NE monsoon

Site ecology

Geographical barrier

ReMarks

uneven topography with strong currents.

scattered reef and mangrove areas along the coast

closer to the land area and depth gradually reduces to the minimum

alluvial marshy tidal flats with a major creek system

4 streams, 2 shoals and a couple of islands visited by flamingoes

river Rukmanathi joins this site

free flushing in and out with ebb and flood.

small islands and reefs in the Marine National Park; weak currents

extreme SE boundary with a tide variation of 7.31 171

high tidal movements and unusually strong currents

presence of tapering land and shoals restrict flood and ebb flows

river run-off less

less commercial fishing operations.

commercial fishing operations active.

only port in Rajkot district with some fishing activities

less fishing except shore based hand and gill net

fishing by trawlers

fishing grounds with increasing efforts

Larval transport in

three

different m

arine ecosystem

s

Table 5.1 Fish egg abundance (in numbers) and site ecology as observed during exploratory surveys during April and Novemeber, 2002

T.

Site Place Fish egg abundance April November 2002 2002

A off 0 114 Okha

B off 60 3177 Bedi

C off 454 60 Navalakhi

D off 35 0 Kandla

E off 0 0 Kukadsar

F off 140 8 Mandvi

00


5.2.4 Redistribution of the fish larvae to nursery grounds

Particles move along with the tidal currents, which is the dominant hydrodynamic force

in the GoK. Since eddies are present in the western region of the GoK, i.e. near the open

boundary, it is possible that these eddies could effectively reduce the flushing rate, and

thus substantially increase the residence time of discharged materials in the GoK [Babu

et al., 2005]. Ebb currents promote less than 10-20% of the accumulated particles to

escape out of the domain along the northern boundary without entering into the eddy

region (Figure 5.2). The particles redistributed along the southern Gulf remained in the

region without being flushed out.

In general, particles released at the GoK mouth were pushed along with flood currents

into the Gulf. This particle dispersion suggests that the fish eggs after hatching may be

dispersed into northern and southern boundaries, rich in reefs and mangroves. However,

during pre monsoon season their entry and exit were partially prevented by the residual

eddies at the gulf-mouth, and this is evident in the particle trajectories (Figure 5.3). The

flow pattern in the GoK can be visualized like a concentric circle with maximum currents

in the mid-gulf and minimum towards the periphery (Figure 4.7). It is observed that

the particles, which are released in the inner Gulf moved further into the eastern Gulf.

Simulation experiment was repeated after releasing the particles during flood and ebb

tides. The transport of particles is highly variable with time and space in accordance with

tides and currents. Across the Gulf and along the Gulf, maximum excursion of particles

is seen during pre monsoon season. In general, it can be inferred that the spawned fish

egg and hatched-out larvae are retained in the GoK domain when released in the interior

stations, close to the eastern boundary of the Gulf.

Trawler catch data at various sampling points suggest abundance of fish in the southern

GoK region (Figure 5.5). This possibly could be explained as an outcome of excess

fish larval retention in the southern gulf. The numerical simulation also corroborates

maximum particle retention in the southern gulf in comparison to the northern gulf. In the

northern Gulf, 10-20% of the redistributed particles escaped along the northern boundary.

In the GoK, spawning sites may vary with seasons, but the dispersal of larvae occurs in


Figure 5.3: Simulation of particle trajectories from various sampling stations: A-off okha, B-off Bedi, C-off Navlakhi, D-off Kandla, E-off Kukadsar, and F-off Mandvi: (a) April 2002 (b) June 2002 and (c) November 2002

50 100 (kilometer)

150


Percentage likelihood retention of lanrE Percentage

MI Above 65 WM so - 65 Li 55-60 ri 50 - 55 IM 45 - 50 IM 40 - 45 MI 35.40 11.1 30 - 35 IN'25-30 I/1 20 - 25

15 - 20 11111 10 - 15

5.10 0 - 5

Below 0

Figure 5.4: Simulation of likelihood of retention of fish larvae in the GoK during: (a) Pre-monsoon, (b) SW monsoon and (c) NE monsoon

such a way that larvae reach nursery grounds in the GoK rather than being dispersed in

the open ocean off GoK.

Even though geographical barriers are imperative in larval retention, their role is super-

seded by the local hydrodynamics in the GoK. The particle dispersions from the same

spawning site showed variations with changes in hydrodynamic conditions (Figure 5.3).

The abundance of fish along the southern region is a clear indication of a driving force

which redistributes the fish larvae to ecologically sensitive mangrove and reef areas in

the GoK. Thus, the model simulation of particle transport in the GoK reiterates the fact

(known biologically) that larval aggregations are going to occur in the southern GoK dur-

ing active breeding phase of fishes with varying dispersal pattern from the spawning sites.

The fish larval abundance in the GoK is qualitatively inferred and the accuracy of mod-

elled particle with the larvae at field level cannot be quantified on real time basis. But, the

results definitely indicate the effectiveness of this tool.

Larval movements of icthyoplankton can be simulated as they are passive, and drift along

with the prevailing currents. But juvenile fishes after planktonic phase cannot be traced

with the help of current movements as they acquire swimming speeds that are able to

counter the currents. Predation, productivity changes, fishing effort and environmen-

tal changes will affect the abundance pattern which is not incorporated in the numerical

model. In protected areas like a gulf, the areas of likelihood of retention of planktonic

larvae may have an impact on the fish abundance as the nursery areas can also become


their possible rearing grounds. But, there are uncertainties involved in this hypothesis.

However, a proper demarcation of potential nursery grounds is definitely an outcome of

these modelling studies (Figure 5.2). Protecting these nursery areas as a part of the fishery

management measure may improve the fish abundance, and also juvenile fish retained in

a productive area tend to remain in the same area. The existing fisheries management

strategy in the GoK is based on demarcation of the ecologically significant sites, which

is not flawless. There are possibilities of nursery grounds shifting from ecologically sig-

nificant sites due to changes in hydrodynamic patterns. This limitation can be rectified

when nursery ground locations are identified based on likelihood of retention areas from

a validated numerical model. This study corroborates not only the ecologically significant

sites prioritized by fishes (Figure 1.3), but also a few additional nursery grounds generated

from the model (Figure 5.2) and confirmed in the field.

The seasonal changes in spawning sites by fish may be an adapted strategy in the GoK to

cope up with the changes in current pattern to advect the eggs from a safe spawning site

to a productive rearing area in the GoK. A lot of uncertainties are involved in explaining

this assumption, but the observations are evident enough to prove this changing pattern.

The calibrations required for a model to estimate the water quality and productivity of a

semi-enclosed coastal environment is complicated as suggested in a previous study along

the western coastal waters of India [Menon, 2004]. The present study invokes the idea that

the demarcation of marine protected areas should be based on the rationale that areas of

maximum likelihood of retention of icthyoplankton will be a better fisheries management

strategy for the GoK, imbibing the concepts of an ecosystem-based spatially structured

approach.


0 0 Trawl catch rate south gulf I Max. observed trawl rates

south gulf Max observed trawl rates north gulf

♦ ♦ ♦ Trawl catch rate north gulf

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Time (months)

Figure 5.5: Trawl catch variation in the northern and southern GoK


5.3 Barnacle larval transport in an estuarine system:

Mandovi-Zuari

53.1 Background

Biofouling by barnacles poses a major threat to navigation, fishing, tourism and port re-

lated activities in the Mandovi-Zuari estuarine system. The life cycle of many fouling and

benthic suspension feeding organisms includes a pelagic larval phase important in disper-

sal. The key factor that influences the final destination of the larvae during dispersal is

their Planktonic Larval Duration (PLD) phase, which depends on both genetic and envi-

ronmental variables. In the barnacles, a number of planktotrophic naupliar stages culmi-

nate into cyprid which is the competent stage for settlement on a suitable hard substratum.

Balanus amphitrite, Darwin, an acorn barnacle, is the dominant intertidal and fouling or-

ganism in the Mandovi-Zuari estuarine system [Anil, 1986; Desai and Anil, 2005]. The

larval phase of Balanus amphitrite includes six planktotrophic naupliar stages, followed

by a pre-settling and non-feeding cypris larva [Anil, 1991; Anil et al., 2001]. A character-

ization based on the quantity of food during planktonic life may indicate the approximate

duration of larval life.

The factors such as larval sinking rate, larval swimming speed and direction and velocity

of ambient currents play a major role in their dispersal. Environmental (e.g. temperature

and salinity) or biological, (e.g. availability and quality of sestonic food or predation)

factors are also imperative in their survival, and influence the length of the larval life.

The dispersion of larvae critically depend on ambient currents, as magnitude of currents

is often larger than their swimming or sinking velocities. The scales of relevance, in both

time and space, are very broad and may include the effects of molecular and turbulent

diffusion, tides, storm-mixing events and wind driven currents, internal waves, meso-

scale eddies and large scale general circulation [A.Okubo, 1994].

In estuaries, freshwater is mixed with seawater by the action of tides, wind effects and

other physical processes. Density differences between seawater and river water may also

result in horizontal pressure gradients which affect the flow patterns [Dyer, 1979]. Con-


cerning larval dispersion in estuaries, the most important physical feature(in both salt

wedge and partially mixed estuaries) is that the bottom water has a net landward move-

ment and surface water seaward when averaged over a number of tides.

The scale of dispersal depends on movement of the water masses, larval behaviour and

duration of pelagic stages. Larval longevity and water movement establish the poten-

tial for dispersal, whereas larval behaviour often determines the actual degree of spread.

Two independent methods have been employed to estimate the quantitative relationship

between duration of the pelagic period and scale of dispersal. Extension of geographic

ranges in species with pelagic larvae and sedentary or sessile adults suggest that a 10 to

15 day larval period results in a spread of over 20 to 30 km. This is in agreement with

oceanic diffusion measurements and the data suggested a positive correlation between

time and spread [Pineda, 2000]. It has been indicated that several hours of larval period

results in spread of the order of hundreds of meters, 1 to 2 days allow dispersal of approx-

imately a kilometer, a week or two increases the scale to 10 km, 1 to 2 months correspond

to 100 km and a year can result in movement of 1000 km. Significantly greater dispersal

requires a substantially longer pelagic period, probably of the order of one week.

In the present case-study, the larval retention and dispersal in the Mandovi-Zuari estuar-

ine mouth was carried out. This study is motivated by the member-vagrant hypothesis

that the larval dispersion and retention pattern in the region are maintained by the areas

(geographical and hydrodynamic) that limit the dispersal and advection of larvae during

the PLD phase. No study has been carried out in the central west coast of India so far

to determine the influence of geographic and hydrodynamic limitations on dispersal or

retention of barnacle larvae. The focus is to find out whether the abundant barnacle popu-

lation in Mandovi-Zuari estuarine system is a manifestation of a closed group trapped by

the hydrodynamic or geographical limitation.

5.3.2 Barnacle larval abundance

Larval abundance was at its highest peak during October 2007 sampling (Figure 5.6).

The abundance significantly differed between all the three sampling periods (Analysis of


Variance (AN OV A) : p < 0.0005). There was an hourly variation in the larval abundance

during May 2007 and October 2007 samplings. During December 2006 sampling, larval

abundance peaked between 2330 and 0230 hours with a non-significant variation between

different sampling hours (ANOVA : p > 0.05). During May 2007 sampling, more larvae

were observed during early morning hours (0230 to 0830 hours) with significant variation

between different sampling hours (ANOVA : p < 0.05). During October 2007 sampling,

two peaks in larval abundance were observed during 1730 and 0130 hours. However,

from the first peak to the second peak the naupliar density was found to be considerably

high. Analysis of variance also showed a significant variation between different sampling

hours (ANOV A : p < 0.05) during this sampling period. A general observation was that

the naupliar density increased with tidal amplitude. The number of cyprids in the study

region was less compared to the nauplii (Figure 5.6).

5.3.3 Barnacle larval dispersion and retention

The results of larval dispersion simulation carried out in the large domain were analysed

for assessing the larval dispersion from the possible spawning locations by calculating the

number of larvae reaching the sampling point. The extent of larval dispersion is influenced

by winds, tides and currents of the respective seasons as described below:

(i) NE monsoon period: Though tide generated currents are dominant, NE winds have

significant role in the larval dispersion and transport during December. A net south-

ward flow along the coast has been observed for the particle transport from all the

spawning locations (Figure 5.7). However, the quantum of particles reaching the

sampling point at Zuari estuary is in the decreasing order from Arambol to Mor-

mugao. There is no effect in the sampling point at Zuari estuary for the particles

released at locations further south of Mormugao Port. The mouth of the Mandovi-

Zuari estuarine system is more exposed to NE winds, hence a net off-shore particle

transport has been observed for the larvae at Singuirim and Dona Paula. Major

amount of particles has been settled along the coastal belt south of Bogmalo for the

spawning locations southward from Singuirim.

0 0

O

3500

3000

2500

2000

1500

1000

500

2

1.5 a

1 cc'

0.5

.0 0 8 -5 -5 -5 2 2 p„ 7, ga 8 CO 0

0 0

0 0 0 CO 0 0 0 0 0 M M C') CD CO C') 0 0

O

-C -C 0 0 0 0

▪

0 O 0 Co)

toM N. co 0)

0 0 0 0 0

1.8

1.6

1.4

1.2

0.8

0.6

0.4

0.2

0

250000

200000

150000

100000

50000

0 C .0

O

-C 0

0 C O

-C C

0.1

C -5 C C 2 4 2 2

.0 -C -C 0 0 0 07 0 0

(NJ C•1 Z7,

0 0 0 0 M co r) CO Q.)

cv

O -C = -C • 2 a a 2 a

cg .0

Nauplii -0-Cyprid -a- Tidal Height 4500

4000

3500

3000

2500

2000

1500

1000

500

1.5

0.5

O = O

▪

0 c> O O oo

▪

0 0 0

2 2 P. 0 0 Fi

0 0

8

-0.5 C

- 2.5

0.5 0

O

C C 0 0 0 0 r


.0

Figure 5.6: Larval abundance noted during the sampling survey in Mandovi and Zuari

130 120 110

ti 100 t 90

80 .s.2 70

60

Time exposure AYS

ligN Above 9 8-9 ri 7-8

Ell 6-7 11111 5-6 11111 4-5 .10 3-4 MI 2-3

1-2 EN 0-1

j Below 0


Figure 5.7: Barnacle larval transport during NE monsoon period along the coastal stretch of Goa


(ii) Pre-monsoon period: Sea breeze is prevalent along the west coast of India during

pre monsoon season [Aparna et al., 2005] and it has significant impact on the diur-

nal cycle of the sea-state off Goa [Neetu et al., 2006]. During May, the predominant

winds are from northwest and the magnitude is of the order of 5-8 m.8 -1 . This re-

sults in a frequent and intense movement of the particles in southward direction and

hence, the spread of the larvae is wide-stretched southwards in comparison to De-

cember (Fig. 5). The magnitude of southward spread of particles is in the increasing

order from Arambol to Galgibagh. However, the quantum of particles reaching the

sampling point at Zuari estuary is in the increasing order from spawning locations

at Anjuna, Singuirim and Arambol (Table 5.2). There is no effect in the sampling

point at Zuari estuary for the particles released at other locations (Figure 5.8).

Time exposure DAYS

1=1 Above 9 8-9

L=I 7-8 11711 6-7 In 5-6 ME 4-5 NE 3-4 ME 2-3 11/111 1-2 1.1 0-1

Below 0

Figure 5.8: Barnacle larval transport during pre monsoon period along the coastal stretch of Goa


(iii) Post-monsoon period: During the first half of October, there exists SW winds with

low magnitude due to the retrieval of SW monsoon. However, during the second

half, NE winds prevail due to the onset of NE monsoon and the particle movement

will be southwards. Therefore, in October dispersal of particle occurs northwards

predominantly in the first half and reverses later in the second half (Figure 5.9).

The settling of the particles inside the Zuari estuary is considerably less, however

it is accumulated near the mouth of the estuarine system. However, the quantum

of particles reaching the sampling point at Zuari estuary is in the increasing order

from spawning locations at Mormugao Port, Bogmalo and Dona Paula (Table 5.2).

Time exposure DAYS

Above 9 1=18-9 C-17-8 17:16-7 .1115-6

-4-5 11.1 3-4

111. 2-3 11.1-2

0-1 ;Below 0

Figure 5.9: Barnacle larval transport during pre monsoon period along the coastal stretch of Goa

There is spatial and temporal variation in the larval dispersal pattern from different spawn-

ing sites. The large domain of study vividly indicates a pattern of northern spawning


points contributing to larval abundance at sampling point during December and May. But,

with the reversing wind and current patterns in October, the larval abundance at sampling

point is supported by southern spawning sites (Table 5.2). Larval transport is southward

during Dec 06 and May 07, depicting the predominant wind direction.

In October 2007 stations northward upto Singuirim show a northward transport but re-

maining stations show N-S transport with predominant transport being northward.

Table 5.2 Distance traversed by modelled larvae from major spawning sites along the coastal waters of Goa during Dec 06, May 07 and Oct 07.

Spawning location Larval dispersal in km (direction) Dec 2006 May 2007 October 2007

Arambol 38(S) 78(S) 22(N) Anjuna 32(S) 64(S) 35(N) Singuirim 25(S) 57(S) 42(N) Dona Paula 25(S) 56(S) 45(N) and 13(S) MPT 17 (S) 53(S) 43(N) and 16(S) Bogmalo 26(S) 46(S) 50(N) and 16(S) Palolem 14(S) 24(S) 10(N) and 6(S) Galgibagh 10(S) 16(S) 12(N) and 2(S)

5.3.4 Modelled and observed barnacle larval retention

The results of the partcles dispersion simulation carried out in the fine resolution domain

(Figures 5.10, 5.11, 5.12) indicate least retention of particles in the estuary during pre

monsoon period. This is in concurrence with the observed field data too. There is higher

abundance of larvae inside the estuaries during NE monsoon and pre monsoon seasons.

The larval movement is corroborated with the tides (Figure 5.6). The simulation indicates

that the particle dispersal is clearly supported by the tidal currents (Figures 5.10, 5.11,

5.12). But some unprecedented high quantity of larval abundance is noticed in the field

measurement during pre monsoon. Less retention of larvae occured in the Zuari estuary.

The particles released from spawning sites moved towards the Mandovi estuary during

NE monsoon and pre monsoons.

101 Larval transport in three different marine ecosystems

12

10

E 8 52 6

4

2

0

12

10

4 8

cg 6

0 0

0 2 4 6 8 10 12 14

2

1

I Conc Ilts/m31(meter)

Above - 1.691e-007

I-7 I 504e-007 - 1.691e-007 IN 5.638e-008 - 7.518e-008

1.316e-007 - 1.504e-007 .1.1 3.759e-008 - 5.638e-008

1.128e-007 - 1316e-007 111.11 1.879e-008 - 3.759e-008

9.397e-008 - 1.128e-007 0 - 1.879e-008

ME 7.518e-008 - 9.397e-008 L Below 0

Figure 5.10: Barnacle larval transport during pre monsoon period in Mandovi-Zuari

12

10

is 8 a)

6

4

2

0 0 2

12

10

E 8

1 6

4

2

0 0 2

12

10

F, 8 a) o 6

4

2

0 0

12

10

E 8 iu E 2 6

4

2

0 o 2 4 6 8

Kilometers 10 12 14

4


511113 Above 1.691e-007 r--1 1.504e-007 -, 1.691e-007 VEIE 5.638e-008 - 7.518e-008

L 1.316e-007 - 1.504e-007 ME 3.759e-008 - 5.638e-008

C:=3 1 218e-007 - 1.316e-007 ME 1.879e-008 - 3359e-008

CM 9 397e-008 - 1.128e-007 0 - 1.879e-008 Below 0

1 1 T_ _ 1_ 1

Larval transport in three different marine ecosystems

103

2 4 6 8 10 12 14

E 0

4

1 Cone 1.kg/m3limeter)

Egn Above 1.691e-007 CA 1.504c-007 - 1.691e-007 L 1 1.316e-007 - 1.504e-007 En 1.128e-007 - 1.316e-007 1.1.

9.397e-008. 1.128e-007

11111 7.518e-008 - 9.397e-008

UM 5.638e-008 - 7.518e-008 OM 3.759c-008 - 5.638e-008

1.879e-008 - 3.759e-008 0 - 1.879e-008

Below 0

Figure 5.12: Barnacle larval transport during pre monsoon period in Mandovi-Zuari


Barnacles are one of the most charismatic invertebrates of the littoral ecosystems. Species

of these groups form large populations of adults in rocky shores and in estuaries that have

the capacity to strongly affect the community structure through spatial interference com-

pletion [Hughes and Griffiths, 1988; Wootton, 1993; Queiroga et al., 2007]. Recruitment

may be broadly-defined as the replenishment of a population with new individuals due to

the process of reproduction and growth. For marine species with indirect development,

recruitment involves several steps: larval development, dispersal during development,

supply to appropriate settlement habitats, settlement and juvenile development [Queiroga

et al., 2007]. The model results indicates that the presence of barnacle larvae in the estu-

ary is more or less controlled by spawning sites in the outer estuarine areas as there is a

clear presence of larvae coming from other sites and settling in the estuary. But, the sites

contributing to this abundance vary seasonally.

The model results (Figure 5.13) for December, May and October are consistent with the

field observations (Figure 5.6), but October month showed unprecedently high observed

values. The quantum of larvae reaching the estuary on the sampling dates (Figure 5.13)

indicates that the seasonal spawning sites are relevant in the abundance of barnacle larvae

at the sampling point. The larval population in the estuary is well-mixed and the geo-

graphical barriers are playing only a very limited role in their retention; hydrodynamics

being the major driving force into their transport.

2001_03 ?8-1§1

Arambol larval concentration Anjuna larval concentration - Singuirim larval concentration - pRaula larval concentration MP larval concentration

9000

8000

7000

cfE6000

—5000 0

1 4000

8 83000

Arambol larval concentration 1 Anjuna larval concentration - Singuirim larval concentration

9000

8000

7000

DPaula larval concentration MP larval concentration 8ogmalo larval concentration

7000

6000

18:00 Time 00

(a)

2000

1000

Figure 5.13: Quantum of larvae reaching the sampling point inside the estuary from major spawning sites during (a) De-cember, (b) May (c) October

Larval transport in

three differen

t marin

e ecosystems


It is likely that the spawning sites may vary remarkably from the modelled sites. Any

stony substratum inhabiting barnacles may also form source of the larvae. It may be

noted that a study carried out in the laboratory showed the minimum age of barnacles to

release larvae as 28 days [Desai et al., 2006]. The active reproduction in barnacles span

over the entire year, with a lull during monsoon. This was observed in a dominant barna-

cle Balanus amphitrite inhabiting the spawning areas [Desai and Anil, 2005]. However,

there is no distinct pattern of spawning over an annual cycle. Maximum percentage of

barnacles, Balanus amphitrite with ripe ovaries was observed during pre monsoon and

early pre monsoon months [Desai et al., 2006] indicating higher density of these nauplii

during these seasons. October being a month supported by spawning sites from south, the

flow apparently supports the larval release from Dona Paula for the months of December

and May.

The relatively higher abundance of barnacle larvae in October (Figure 5.6) can also be

attributed to other factors such as rainfall, location of spawning and stock abundance,

which are not considered in the present study. Unprecedented precipitation followed by

a sunny day, and spawning from a site other than the rocky shore like a cement pillar

or anchored vessel may give rise to erroneous comparison. This indicates the relevance

of repeated time-series sampling to confirm the larval transport process. The physical

observations in the study region during October indicate favourable wind and current

patterns corroborating the view that more barnacle larvae are transported to the estuary

from the southern sites (Figure 5.14). Turbulent tidal flow forces the larvae to undergo

mortality, however this depends upon the type of larvae. Barnacle nauplii and cyprids

have no effect on turbulent tidal flow compared to larval forms of mollusks (veligers)

[Jessopp, 2007].

Naupliar development occurs in pelagic surface to subsurface waters of the shelf, and

mostly consists of naupliar and cyprid stages. The release of larvae mostly coincides

with high tide, and newly released nauplii and subsequent stages depend on flow of water

(Figure 5.6). The modelled and observed pattern of larval dispersion along the coast is

similar in December and May. Qualitatively we are able to interpret their abundance in a

region, based on their dispersion characteristics. Larvae are accumulated in the spawning

Wind Speed [m/s]

Wind speed (m.s-1)

Above 10 NM

8-10

111. 6 - 8 .11 4- 6 - MI 2 - 4 NO 1 - 2 1 I Below 1


00:00 00:00 00:00 2007-09-25 10zate and timel 0-15

00:00 10-25

Current speed [m/s]

Cu

rren

t sp

eed

(m.s

-1)

1.0

111{1111 00:00

2007-09-25

•

00:000 Date and time

Yl 0.5 -

.0

Figure 5.14: Prominent wind and current direction during the unprecedented larval abun-dance during pre monsoon


site near Dona Paula during October (Figure 5.6). The abundance is comparatively less

in May than December with a wider dispersal pattern. In October, a sampling away from

the dispersal core reflects a lesser abundance as the larvae may not be spreading to this

point. The dispersal core or spawning site is very relevant in this month as dispersal zone

is narrow. Therefore, a slight change in the spawning site will reflect a major change

in the larval availability at a particular point. The simulated tide and current (Table 4.2)

explains the possible transport of larvae along with changing hydrodynamics. Tidal fronts

which have been correlated to larval accumulation occur in shallow and not in deep waters

[Epifanio, 1988; Clancy and Epifanio, 1989]. It can be pointed out that larval return to

open-coast habitat can potentially occur passively by eddy diffusion from an offshore

larval pool to shallow water, semi-continuous advection and advective transporting events

which may involve vertical swimming behavior, while active transport can potentially be

achieved through swimming shore-ward [Pineda, 2000].

Larval abundance in the estuary significantly increases with tide amplitude as flood cur-

rents push more larvae to the estuary. Compared to larvae of organisms living in the open

coast, estuarine organisms still have to solve an extra problem after having been trans-

ported to the near shore environment, that is finding out estuarine inlets and travel stream.

The currently accepted view is that invertebrate and fish larvae migrate into estuaries us-

ing selective tidal stream transport [Forward-Jr and Tankersley, 2001]. During upstream

selective tidal stream transport, larvae settle on or move close to the bottom during ebb

tide to avoid being displaced seaward or ascend in the water column during flood tide

[Queiroga et al., 2007].

An offshore sampling of barnacle larvae outside the estuarine domain clearly indicated a

less abundance of larvae. The average abundance of larvae which was collected from

offshore domain during 3 different sampling periods was 474 (+664) larvae per 100

m3 , which is much lower than the larval abundance within the estuary. Alvarez et al.

[1990] illustrated through their study that along-shore diffusion was on an average ten

times stronger than the cross-shore diffusion. This corroborates our numerical experi-

ments that the larval spread is only in the coastal and estuarine waters unlike a pelagic

offshore larval pool in the case of Californian barnacle [Pineda, 2000]. Non-decapods


marine invertebrate larvae are generally small and have limited swimming ability [Chia

et al., 1984], and therefore those larvae must probably be transported back by diffusion or

advection [A.Okubo, 1994]. It is also suggested that swimming is unimportant for most

of the marine invertebrate species •[Shanks, 1995], although it is a possibility for certain

larger decapods larvae and other large short lives larvae [Olson, 1985; Pineda, 2000].

5.4 Aggregation pattern of fish larvae an open coastal

stretch: off Mangalore

5.4.1 Background

Fish landing data shows that Mangalore contributes to about 40% of the total fish landing

of Dakshina Kannada District. In the coastal villages, about 75% of population is engaged

in fishing related activities. Fishing operations are carried out using purse-seines, otter

trawls, and long-lines with mechanised boats, whereas non-mechanised boats are used

for gill netting or cast nets and other small scale fishing activities.

Experimental trawling undertaken in the study area have shown that the area is dominated

by a variety of economically important finfish and shellfishes such as portunid crabs, flat

fish, squids Loligo sp., peraeid prawns, pomfrets sciaenids etc. These observations clearly

indicate that the fisheries play an important role in the economy of this region. Being an

industrial area, the spawning, migration and breeding activities of the fishes are widely

affected in the Mangalore coastal region. For this purpose, it is necessary to have baseline

information on the occurrence of fish eggs and larvae and spawning activities of the fishes.

5.4.2 Spawning behaviour of fish/shellfish larvae in the Mangalore

coastal region

1. Shell fishes in the vertical haul: Vertical distribution data of decapod larvae off

Mangalore showed that the Lucifer sp. dominated in the study area. It was well rep-


resented throughout the study area with density ranging between 435 and 6958 per

100 m3 . Larval stages of Acetes sp., occurred during all sampling occasions and its

abundance ranged from 435 to 4784 per 100 m 3 . Penaeid larvae were represented

by commercially important prawns such as Metapenaeus dobsoni, Metapenaeus

affinis and Penaeus merguiensis. Among carideans, only larval stage of Palae-

mon sp. was represented in this area. Callianassid larvae were not common. The

brachyuran zoea were not well represented except during March and April. The

groups that occurred in vertical hauls were encountered in horizontal and oblique

hauls. Larval stages of Lucifer sp. dominated. Acetes sp. was relatively uncommon.

Penaeid larvae were represented by the same species collected in vertical hauls, but

their density was relatively lower. Carideans were represented by larval stages of

Alpheus sp. as against a sample representation of Palaemon sp., Brachyuran zoea in

both hauls were also collected in a few samples in small numbers, while porcellinid

zoea was collected in oblique hauls.

The compilation of data [Verlecar et al., 1998] and previous records [Rivonker et al.,

1990] clearly indicate an average abundance of peanaeid shrimps in the water col-

umn. This indicate the relevance of taking up fish eggs and larval abundance studies

in the region.

2. Fish eggs and larvae: The collection of fish eggs and larvae were carried out along

with zooplankton samples. In the vertical haul, fish eggs and larvae ranged from

0 to 4711 and 0 to 236 per 100 m 3 . The fish eggs and larvae occurred in large

numbers during September and November, 1997 and February, 1998 while fish

larvae were mainly observed in September and October, 1997 and March, 1998. In

the horizontal hauls, fish eggs and larvae were poorly represented while in oblique

hauls, fairly good numbers comparable to vertical hauls were recorded. Sufficiently

large number of fish eggs and larvae were observed in October 1998 in oblique hauls

while fish larvae were almost absent during this month. Comparison of fish eggs and

larvae in different periods showed that fish eggs and larvae are present throughout

the study period [Verlecar et al., 1998], but their numbers vary depending on the

season. Monthly values showed that decapod larvae were much higher than the


fish eggs and larvae at both locations. Fish eggs and larvae and decapod larvae

in combination form a maximum of 4% of the total zooplankton abundance off

Mangalore. The percentage composition of fish egg, larvae and decapod larvae

does not show significant differences (p > 0.05 , ANOV A). These observations

indicate that Mangalore region can be a prospective fishing ground, irrespective of

the season, but can be a prospective nursery ground too.

3. Spawning season: One of the remarkable characteristics of fishes is that they have

high fecundity. However, the survival of fish eggs, fish larvae and decapod larvae

depends on the predators and the surrounding environment. The eggs of majority

of bony fishes are of same size, planktonic in form and hatch into larvae of about 3

to 6 mm in length. The larvae are transparent, with prominent dark eyes, large yolk

and they swim by wriggling movements throughout the water column. Because of

this behaviour, large differences were observed in number of fish eggs and larvae in

different hauls while their numbers were almost steady in vertical hauls. Feeding by

these larvae takes place during dusk and dawn. Their first food is often large algal

cells but they also rely on copepod nauplii or larval stages of molluscs. During

the end of larval life as zooplankton, metamorphosis takes place making the larvae

transparent, fins and scales develop and they change their shape into that of little

fishes. When fish metamorphosise they are ready to settle in a nursery ground,

usually along the coast or estuary. The migration from spawning ground to nursery

is called the larval drift. So, irrespective of the season a larval drift study in the

Mangalore region will improve the understanding of the prospective nursery areas

for Mangalore region.

5.4.3 Effect of physical process and biological abundance on fish lar-

val transport in the Mangalore coastal region

Particle trajectories for different spawning sites have been simulated as field studies are

not revealing any clear pattern of abundance of egg/ larvae along the coastal waters off

Mangalore. The field observations indicated that the larval abundance is high without any

Time exposure DAYS

:71 Above 9

1 8 - 9

1 7 - 8 6 - 7

—1 5 - 6 4 - 5

71 3 - 4 MI 2 - 3

1 - 2

11111 0 - 1 —1 Below 0

, ,

30 35 40 45 40 45

30 35 40 45 (kilometer) (kilometer) (kilometer)

12/31/06 00:00:00 04/18/07 00:00:00 10/18/07 00'.00:00


specific spatial variation. The particle trajectory plot was employed to see the possible

dispersal of larvae from different coastal locations to understand the possible mechanisms

influencing larval dispersion in the region. The larval tracks are having a uniform pattern

except for their dispersal at the south boundary of the Mangalore domain (Figure 5.15).

This is in concurrence with earlier field studies [Rivonker et al., 1990] which have shown

that the fishing grounds are spread over the region without any spatial changes seasonally.

Like zooplankton, initially larvae live as plankton (drifting in water) and they make daily

Figure 5.15: Simulation of particle trajectories from three stations along the coastal waters of Mangalore during pre monsoon, post monsoon and NE monsoon

vertical migrations from below the euphotic layer towards the surface. After attaining

certain growth, fishes tend to move off slowly into somewhat deeper waters and thus


movement has been described as a horizontal diffusion away from the shore. During

this period of their life cycle, they feed on local foods such as copepods, bivalves and

worms. Their speed into deeper waters gradually leads them to grounds where adults live

and gradually they are recruited to adult stocks. Length-weight analysis, maturity and

spawning behaviour showed that the fishes off Mangalore belong to one year or less than

one year year-class [Rivonker et al., 1990]. Most of the fishes were immature, and sex

could not be identified. Commercial fish species during their adult life, mostly migrate

every year from feeding grounds to spawning grounds and back again. The spawning

season may last for few days or few months for particular species. The eggs take up water

as they are released and spawn in batches. During this study period [Verlecar et al., 1998],

not a single fish having mature ovary was detected. It can be stated that spawning takes

place in offshore waters and the eggs as well as larvae could get drifted away by tides and

currents to the coast where the eggs hatch out into larvae and mature into young fish. Thus,

Mangalore coast having rich phytoplankton and zooplankton biomass could act as a good

nursery ground for the commercially important fishes as evident from the predominant

zero-year (i.e. < 1 year) size-class of the fishes, fish egg and larval distribution.

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